1. Field of the Invention
The present invention relates to a compound semiconductor device, and more specifically to a chalcogenide compound or gallium nitride based compound semiconductor light emitting device and a method of manufacturing the same device.
2. Description of the Prior Art
Recently, as semiconductor light emitting devices for emitting light in the wavelength range between green and blue, light emitting devices using ZnSe based materials of the II-VI group compound semiconductor or using GaN based material of the III-V group compound semiconductor are now intensively being researched and developed. In the recent research level, some light emitting diodes or semiconductor laser devices have been manufactured by way of trial in both the cases of ZnSe and GaN based materials
In more detail, in the case of the II-VI group compound semiconductor, ZnSe based material of zinc chalcogenide is now being developed. Further, in the recent research level, its crystal growth has been realized in accordance with an MBE (molecular beam epitaxy) method. However, the research level described above is far from a production level. Further, when a metal-organic chemical vapor deposition (MOCVD) method is adopted, since it is difficult to realize a p-type conductive semiconductor layer, there is little prospect for realization of the light emitting devices using the same material.
On the other hand, in the case of the GaN based semiconductor, although some light emitting diodes have been already realized in accordance with the MOCVD method, the semiconductor laser device has not yet reached the product level which can be put on the market. This is because the resistance components not related to the light emission is relatively large, with the result that the quantity of heat generated by the laser element is large and thereby the lifetime thereof is short. Further, in the case of the GaN based light emitting devices, since the melting point of the crystal thereof is relatively high, there exists a difficulty in crystal growth. In addition, the crystal is not easily etched by use of acid or alkali, it is very difficult to process the element thereof.
In this specification, [gallium nitride based semiconductor] or [GaN based semiconductor] implies all the semiconductors having compositions obtained by changing the composition ratios of x and y in each range of the chemical formula expressed by In.sub.x Al.sub.y Ga.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1). For instance, InGaN (x=0.4, y=0) is also included in [gallium nitride based semiconductor].
Here, in the conventional GaN based semiconductor laser, the manufacturing process is as follows: First, an n-type GaN contact layer, an n-type AlGaN cladding layer, an InGaN active layer, a p-type AlGaN cladding layer, and a p-type GaN contact layer are deposited or laminated in sequence on a sapphire substrate. Then, a p-side electrode is formed on a p-side surface thereof. In addition, a part of the n-type GaN layer is etched by dry etching method, to form an n-side electrode.
Further, in addition to the above-mentioned process, a trial of reducing the laser oscillation threshold current has now been made by injecting electrons into the part of the active layer in a high density. In this process, an insulating layer is formed just under the p-side electrode to narrow the current path, or else an internal current-blocking structure is formed by dividing the crystal growth process into two times.
In the conventional light emitting devices of ZnSe based semiconductor realized in accordance with the above-mentioned MBE method, a serious problem exists in that the device reliability cannot be yet secured. It is considered that this problem results from the GaAs substrate and the p-type electrode. In more detail, since the lattice constant is slightly different between GaAs and ZnSe, when current flows, the mutual diffusion of the composing elements is further promoted and thereby the diffused elements interact with each other as dopant, with the result that a high resistance layer is inevitably formed in the interface between the two layers. Further, in the same way as with the case of the afore-mentioned GaN, since a good ohmic contact has not been yet realized at the electrode contact, a relatively high voltage is applied to the electrode portion when current flows, so that there exists a problem in that the device deteriorates in an extremely short time due to the heat generation thereat.
In the case of the II-VI group compound semiconductor, the method of doping p-type impurities has not yet been established well in the MOCVD method. In more detail, although the adding method can be attained partially by use of lithium (Li) of the I group, the carrier concentration is now as low as about 10.sup.16 cm.sup.-3. When ZnSe is used as the substrate, the problem related to the interface can be solved. In this case, the iodine method is now considered as a method of causing the least crystal defects in the substrate. However, if the substrate obtained by the iodine method is adopted and Ga of the III group element is used as the additive impurities in the first layer on the substrate, there so far exists a problem in that the substrate resistivity increases due to the diffusion of the additive impurities. A photo-excitation MOCVD method by using a light source having light emission wavelength less than 200 nm (e.g., excimer laser or mercury lamp, etc.)has been adopted for relatively long time. However, since the roughness of the grown surface is prominent, the method cannot be used from the practical standpoint.
On the other hand, in the case of the GaN based compound of the III-V group semiconductor, there exists such a tendency that the resistivity of the p-type contact increases with increasing band gap. Therefore, a serious technical problem is how to reduce the resistivity of the p-type contact in order to secure the reliability of the semiconductor laser. In other words, since a large contact resistance exists between the p-type GaN contact layer and the p-side electrode, a large voltage drop is inevitably produced at this contact portion, thus resulting in an unstable laser operation or unstable laser oscillation due to the heat generation. To overcome these problems, Japanese Published Unexamined (Kokai) Patent Application No. 8-330629 discloses such a method of introducing a large amount of dopant (e.g., Mg) near the surface of the p-type contact whenever the p-type contact is formed.
In this method, even if Mg is added, the attainable maximum carrier concentration is as low as about 10.sup.18 cm.sup.-3 and the activation ratio (i.e., the proportion of the effective number of carriers to the added impurities) is as extremely low as 1 percent. In addition, even if the impurities are added excessively beyond the above value, the carrier concentration is saturated and the activation ratio decreases abruptly beyond a certain value. Under these circumferences, even if the impurities are added to the compound semiconductor having the stoichiometric composition, there still exists a problem in that it has been difficult to form an ohmic contact of sufficiently low resistivity.
On the other hand, in order to reduce the threshold current of the laser oscillation, it is necessary to adopt the internal current-blocking structure as already explained. However, in the case of the GaN based semiconductor laser having the internal current-blocking structure using SiO.sub.2 as the current blocking layer as disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 4-242985, a difference in thermal expansion ratio between SiO.sub.2 and GaN causes leak current at the interface thereof, thus resulting in a problem in that the device performance deteriorates. There exists a trial of using a semi-insulating GaN or n-type GaN as the current blocking layer, as disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 8-975071. However, since the current cannot be reduced sufficiently by using those GaN layers, a laser diode having low threshold current has not yet been obtained.